*2.2.1. Stress on birds*

Stress is a condition of vulnerable homeostasis and is affected by management and environment factors. Birds normally have limited resources in the body for growth, response to environment changes and maintain a defense system for diseases. The stress on birds can be due to a number of factors including cold stress, heat stress, high humidity, transportation stress, intensive farming, high stocking density, overcrowding, low per bird space, decreased ventilation, poor litter conditions, accumulation of bad smell in sheds and poultry houses, off feeding, water deprivation, poor management, bad sanitary conditions, very wet or extremely dry litter, dusty environment, parasitism, nutritional deficiency, fever, and so on. In these cases, there can also be vaccine failure in livestock. The poultry birds are sensitive to both cold and warm weather [3]. Heat stress is an important factor of economic loss for the producer [19], while cold stress modifies the immune response of broilers [20]. The symptoms of stress in birds include panting, increased thirst, reduced appetite, reduced egg production, decreased weight gain, small sized eggs, thin egg shells, reduced growth, prostration, etc. All the factors including management conditions, substandard hygienic conditions, etc. contribute to the possible causes of high economic losses by leading to vaccine failure [21].

#### *2.2.2. Concurrent disease*

It is highly important that the vaccination should be done in healthy birds. The vaccination in sick and diseased birds may not provide fruitful results; rather, vaccine reaction may occur leading to extra stress and an increased morbidity and mortality rate. Moreover, any other disease condition may also contribute to vaccine failure. When the birds are morbid due to the same disease for which vaccination had been done, then there will also be vaccine failure because the antibodies produced against the pathogenic agent will neutralize the antigen of vaccine and a reaction may take place in the body of birds and vaccination may worsen the condition of disease.
